1. Field of the Invention
The present invention relates to a method of etching a magnetic material and the like and, more particularly, to a method of etching a spin tunneling magnetoresistive film and the like.
2. Related Background Art
In recent years, MRAM (Magnetic Random Access Memory) which is a high speed nonvolatile memory has been attracting attention as a memory which replaces many solid memories which are at present in use. In particular, MRAM which utilizes the spin tunneling magnetoresistive effect is favorable for high recording density design or high speed read because large read signals are obtained, and the possibility as MRAM has been demonstrated in recent study reports.
The basic structure of a magnetoresistive film used as an element of MRAM is a sandwich structure in which magnetic layers are formed in an adjacent manner via a nonmagnetic layer. However, when the element size is reduced in order to increase the recording density of MRAM, MRAM which uses an in-plane magnetized film cannot retain information due to the effects of a diamagnetic field or the curling of the magnetization of an end surface, thus posing a problem. This problem can be avoided, for example, by forming a magnetic layer in the shape of a rectangle. Under the method, however, the element size cannot be reduced and hence a great improvement in recording density cannot be expected.
Therefore, as described in the Japanese Patent Application Laid-Open No. 11-213650 (U.S. Pat. No. 6,219,275), for example, a suggestion has been advanced that the above-described problem is to be eliminated by using a perpendicular magnetized film. According to this technique, a diamagnetic field does not increase even when the element size decreases and, therefore, it is possible to realize a magnetoresistive film having a size smaller than that of MRAM which uses an in-plane magnetized film.
An amorphous alloy film of a rare earth metal and a transition metal is advantageously used as a perpendicular magnetized film used in MRAM. For example, Gd and Tb can be used as a rare earth metal and Fe and Co can be used as a transition metal.
Usually, in the MRAM fabrication process, a selective transistor is first formed on a Si wafer. After that, a film of a TMR element is formed on the whole area of the Si wafer and a memory element is fabricated by the microfabrication of the multilayer film.
In the semiconductor process, reactive etching is used during fabrication. Reactive etching is an etching process which involves holding a workpiece, for example, a Si wafer in a vacuum chamber and converting Si into SiF4 etc. by causing a reactive gas such as F2 to react with Si in a plasma, thereby removing Si. In reactive etching, the etching rate is high, a removed substance will not adhere again to a side wall of the workpiece, and microfabrication is possible.
However, in the etching of MRAM which uses a magnetoresistive film in which a magnetic film contains a substance having a higher melting point than Si, it is difficult to prevent a removed substance from adhering again to a workpiece. For example, in a case where a TMR element which uses a rare earth metal-transition metal alloy film is etched, for example, by using Ar, which is an inert gas, as shown in FIG. 6, a sidewall substance 103 which comprises a rare earth metal-transition metal alloy adheres again to a worked sidewall of a TMR element, which comprises a first magnetic film 111 formed on a substrate 101, a tunnel insulating film 112 and a second magnetic film 113. In a magnetoresistive film in which a current is caused to flow in a direction perpendicular to the film plane, the first magnetic film and the second magnetic film short electrically due to the formation of a sidewall substance on a sidewall of the electromagnetic film, and it becomes impossible to obtain the magnetoresistive effect.
As a method for solving this problem, as described in the Japanese Patent Application Laid-Open No. 2003-78184, there has been proposed a method which involves changing a sidewall substance into another sidewall substance having high electrical resistance by using N2 or a mixed gas of an inert gas and N2 as an etching gas. For example, as shown in the graph of FIG. 2(a) of Japanese Patent Application Laid-Open No. 2003-78184, in a case where a TMR element which uses a rare earth metal-transition metal alloy film is etched by using an inert gas alone and worked to a size of 20 μm×20 μm, the resistance value was tens of ohms. However, the resistance value can be increased to tens of kilo-ohms by using N2 gas.
It is necessary to reduce the cell area in order to increase the recording density of MRAM. In a magnetoresistive film in which a current is caused to flow in a direction perpendicular to the film plane, the resistance value of a memory element is proportional to the area. That is, if the area of an element of 20 μm×20 μm is reduced to, for example, 20 nm×20 nm, then the resistance value is expected to increase to 106 times. At this time, unless a sidewall substance has sufficiently high resistance, a current which flows via the sidewall substance increases in association with the scale down of the element and a sufficient magnetoresistive effect cannot be expected.
Furthermore, when the film thickness of a magnetic film is relatively large, the thickness of a sidewall substance adhering to a side surface of the magnetic film also increases. For this reason, also in this case, a current flowing via the sidewall substance increases similarly and it is feared that a sufficient magnetoresistive effect could not be obtained.
Therefore, the present invention has as its object the provision of a method of etching a magnetic material in such a manner as to suppress a current flowing via a sidewall substance to a greater extent, and the like.